VGLUT1 synapses and P‐boutons on regenerating motoneurons after nerve crush

Abstract

Stretch-sensitive Ia afferent monosynaptic connections with motoneurons form the stretch reflex circuit. After nerve transection, Ia afferent synapses and stretch reflexes are permanently lost, even after regeneration and reinnervation of muscle by motor and sensory afferents is completed in the periphery. This loss greatly affects full recovery of motor function. However, after nerve crush, reflex muscle forces during stretch do recover after muscle reinnervation and reportedly exceed 140% baseline values. This difference might be explained by structural preservation after crush of Ia afferent synapses on regenerating motoneurons and decreased presynaptic inhibitory control. We tested these possibilities in rats after crushing the tibial nerve (TN), and using Vesicular GLUtamate Transporter 1 (VGLUT1) and the 65 kDa isoform of glutamic acid-decarboxylase (GAD65) as markers of, respectively, Ia afferent synapses and presynaptic inhibition (P-boutons) on retrogradely labeled motoneurons. We analyzed motoneurons during regeneration (21 days post crush) and after they reinnervate muscle (3 months). The results demonstrate a significant loss of VGLUT1 terminals on dendrites and cell bodies at both 21 days and 3 months post-crush. However, in both cellular compartments, the reductions were small compared to those observed after TN full transection. In addition, we found a significant decrease in the number of GAD65 P-boutons per VGLUT1 terminal and their coverage of VGLUT1 boutons. The results support the hypothesis that better preservation of Ia afferent synapses and a change in presynaptic inhibition could contribute to maintain or even increase the stretch reflex after nerve crush and by difference to nerve transection.

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

“…After nerve transection at the same location EPSP amplitudes from injured and regenerated Ia afferents onto heteronymous uninjured motoneurons recovered to around 50% of their original size (Mendell et al, 1995) while homonymous Ia EPSPs recovered much less in injury models affecting both Ia afferents and motoneurons (Eccles et al, 1959). These results agree with the good recovery of VGLUT1(Ia/II) synapses after nerve crush in rats (Schultz et al, 2017) compared to the lack of recovery when transecting the same nerve (Alvarez et al, 2011). Interestingly, recovered VGLUT1 (Ia/II) synapses following nerve crush displayed reduced presynaptic GABAergic P-boutons (Schultz et al, 2017), suggesting decreased presynaptic inhibition.…”

“…A major problem was the lack of anatomical analyses of Ia synapses after nerve injuries. These did not occur until recently (Alvarez et al, 2010(Alvarez et al, , 2011Rotterman et al, 2014;Schultz et al, 2017) facilitated by the discovery of VGLUT1 as a marker of proprioceptive synapses in the ventral horn (Todd et al, 2003;Alvarez et al, 2004). VGLUT1 synapses in lamina IX and on motoneurons, are mostly Ia synapses, but a minority might also originate from type II afferents (Alvarez et al, 2011;Vincent et al, 2017).…”

Spinal motor circuit synaptic plasticity after peripheral nerve injury depends on microglia activation and a CCR2 mechanism

“…On the other hand, deletion of Ia/II inputs might reduce connectivity incongruences between proprioceptors and MNs inside the spinal cord after both sensory and motor axons regenerate in the periphery. Different types of nerve injuries result in axon regeneration with variable success on targeting the original muscles, which correlates with more or less loss of Ia afferent synapses (Schultz et al, 2017). CCR2 cell entry similarly fluctuates with injury severity.…”

“…The number of VGLUT1 terminals per cell body was obtained and their density calculated by dividing the number of VGLUT1 contacts by the total cell body surface (per 100 m 2 ). VGLUT1 contacts on dendrites were estimated as linear densities by dividing the number of VGLUT1 terminals by total dendritic length of the traced dendrites (per 100 m) Schultz et al, 2017). Sholl analysis was used to examine VGLUT1 distributions in dendrite segments at 25 m bins of incremental radial distance from the center of the cell body.…”

Botulinum Neurotoxin Application to the Severed Femoral Nerve Modulates Spinal Synaptic Responses to Axotomy and Enhances Motor Recovery in Rats

“…Synaptic terminals are removed from cell bodies and dendrites of motoneurons by activated microglial and astroglial cells [ 1 – 6 ]. The overall posttraumatic loss is reversed to a large extent if muscles become reinnervated [ 3 , 6 , 7 ], but restoration of some synaptic inputs is incomplete [ 8 – 11 ]. Such deficits, for example, in cholinergic and glutamatergic innervation, may contribute to functional deficits after muscle reinnervation as they are well correlated with functional performance after long-term reinnervation [ 9 , 12 ].…”